Human Immunodeficiency Virus type 1 (HIV-1) is responsible for the majority of cases of HIV infection and AIDS throughout the world. It was thought to be the sole causative agent until 1986, when a second type of HIV was isolated, namely HIV type 2 (HIV-2).
HIV-2 is largely confined to West Africa but sporadic reports of HIV-2 infections have been made in several countries outside this area such as, for example, in Portugal, France, Mozambique, Angola, India and Brazil. However, due to antibody cross-reactivity to the proteins of HIV-1 and HIV-2, patients with HIV-2 infection may receive a diagnosis of HIV-1 infection and it is likely that the number of HIV-2 infections around the world is largely undercounted.
HIV-1 and HIV-2 are 50% similar at the genetic level with about 60% homology in conserved genes such as gag and pol, and about 39 to 45% homology in the envelope genes (Guyader et al., 1987). They share many features such as the route of transmission or the infected cell types but also maintain some distinct characteristics.
Although HIV-2 is less transmissible than HIV-1 and causes infections that progress more slowly, it can also lead to immunosuppression and clinical AIDS. Correct identification of HIV-2 infection is important because clinical management and treatment regimens differ for HIV-1 and HIV-2 infections. Indeed, some antiretroviral drugs, such as non-nucleoside reverse transcriptase inhibitors and some protease inhibitors, have a reduced efficacy in the treatment of HIV-2 infection (Ntemgwa et al, 2009).
The standard procedure for laboratory diagnosis of HIV infection usually consists of performing an HIV antibody immunoassay (a third or fourth-generation enzyme immunosorbent assay (EIA) or a rapid simple test) which, if reactive, is followed by confirmatory tests (Western blot or immunofluorescence assay). Western-blot (WB) or immunoblot is the most widely used confirmatory test. However, these tests are expensive, time-consuming and can yield indeterminate results due to HIV-2 antibody cross-reactivity to the proteins of HIV-1.
Based on the availability of new HIV tests, a new algorithm was proposed in 2010 in the USA to replace this standard procedure (Pandori and Branson, 2010). This algorithm includes a highly sensitive HIV-1/2 immunoassay such as a third or fourth-generation EIA, which, if reactive, is followed by a highly specific HIV-1/HIV-2 differentiation immunoassay. Specimens that are reactive for antibodies on both tests would be considered positive for either HIV-1 or HIV-2 antibodies. Specimens negative for antibody on the second test would then be tested with a nucleic acid amplification test (Delaney et al., 2011). This new algorithm was tested and was shown to outperform the previous algorithm because it was more sensitive for detecting HIV-1 infection, provided a great number of definitive results and detected HIV-2 more efficiently (Styer et al., 2011).
To date, several assays are available to differentiate HIV-1 from HIV-2 infections such as for example the Multispot® HIV-1/HIV-2 Rapid Test (Bio-Rad Laboratories), the Recombigen® HIV-1/HIV-2 RTD test (Cambridge Biotech), the PEPTI-LAV® 1-2 test (Bio-Rad Laboratories), the INNO-LIA® HIV Confirmation test (Innogenetics) or the Immunocomb® II HIV-1 & 2 BiSpot test (Orgenics). The INNO-LIA HIV I/II Score assay has been approved for diagnostic use in the European Union but currently cannot be used in the US. To date, only one assay that is capable of differentiating HIV-1 from HIV-2 infections has been approved by the FDA, namely the Multispot® HIV-1/HIV-2 Rapid Test (Bio-Rad Laboratories). This flow-through test differentiates HIV-1 from HIV-2 antibodies by using a synthetic peptide representing the immunodominant epitope of the HIV-2 virus gp36 envelope glycoprotein, a recombinant gp41 (HIV-1) envelope glycoprotein and a synthetic peptide representing the immunodominant epitope of the HIV-1 virus gp41 envelope glycoprotein. However, with these assays, a significant proportion of specimens remain undifferentiated. Furthermore, the Multispot® assay often requires an error-prone and time-consuming dilution.
Therefore, there remains a need for a simple, quick and cost-effective diagnostic test providing highly sensitive and specific results to differentiate HIV-1/HIV-2 cross reactivity from HIV-2 true reactivity and thus reducing the percentage of indeterminate specimens. This test should be suitable for use in multi-test algorithms designed for statistical validation of rapid HIV test results.